Aircraft icing is a dangerous condition. The formation of ice on the exposed leading edges of wing components changes the aerodynamics of the aircraft and may lead to a decrease in lift and an increase in drag. Typically, the ice formation occurs in clouds at altitudes less than 20,000 ft. (Above this level, the water molecules have already been transformed to ice and will typically just bounce off the foil.) When an aircraft flies through an ice formation zone—i.e., clouds with high moisture content below 20,000 ft. —ice will deposit on the aero-foil. It is critical that excess ice deposits are detected and removed before they cause aerodynamic and mechanical malfunctions and reduce fuel consumption.
Over the years there have been many attempts to design suitable ice detectors based on varying concepts, including, for example, monitoring the conductivity of an exposed rod and monitoring the frequency change of an exposed vibrating element. These electrical and electro mechanical approaches are subject to significant false readings and require continuous maintenance.
Of particular interest herein are optical ice detection approaches. For example, U.S. Pat. No. 6,010,095 discloses a device for sending a beam of light parallel to a surface being monitored for ice. If there is no ice accretion, the beam is reflected back to a light detector. If there is ice accumulation with a height sufficient to interfere with the emitted light beam, the ice will cause a diffraction and re-direction of the beam, thus preventing it from reflecting back to the detector. This device will determine ice accumulation only if it has reached a certain point. Another optical detector configuration is described in U.S. Pat. No. 7,586,419, which comprises an array of optical sensors, which are mounted flush with an outer surface of an aircraft skin, and a centrally located optical emitter. Light is emitted by the emitter outwardly of the aircraft surface, and the layer of accreted ice diffuses the emitted radiation scattered and reflected. The diffused radiation is detected by the sensors, and the spatial distribution of the detected intensity diffused radiation about the emitter can be used to calculate the thickness of the layer of ice and the type of ice. This device requires that the optical sensors be mounted flush with the surface being monitored, which may require drilling or otherwise modifying the aircraft surface. Obviously, any structural modification to a wing component is undesirable as it may compromise the function or structural integrity of the wing component.
Therefore, there is need for a simplified, rugged optical sensor design, which can determine both the presence and the thickness of ice on a monitored surface without modification to the monitored surface. The present invention fulfills this need among others. Indeed, the sensor of the present invention can be applied to measure the level of accumulation of any material, providing the material has a refractive index different from its environment.